A mobile telecommunication device such as a cellular telephone, portable computer, or radio handset, has at least one radiator formed by an electrical conductor for transmitting and receiving communications data and having a resonant frequency that is tuned to a specific band. Such a radiator is mounted at least partially inside a housing together with at least one other housing part for the communications device.
Such antennas are known from the prior art and are constituted for example as rod antennas, helical antennas, or monopole or dipole antennas, as well as PIFA's (planar inverted f-antennas) in various telecommunication devices. Modern telecommunication devices, whether cell phones, portable computers such as notebooks or PDA's (personal digital assistants), radio handsets, or the like, are being designed in increasingly smaller sizes to enhance the convenience of such devices. At the same time, such telecommunication devices incorporate a continuously increasing number of functions.
In particular cell phones now serve many different functions. They may now be used to take photographs, play music, receive e-mail, access internet services, etc. The integration of more and more functions with the simultaneous miniaturization of the devices imposes high demands on the individual components, in particular their size.
Among other components, the antennas of such devices have increasingly become the object of specific improvements, in particular structural miniaturization. However, this must be balanced with the requirement for the best possible transmitting and receiving power, also in multiple frequency bands.
A further problem for antennas for telecommunication devices is the alteration of the antenna's characteristics by the device's housing. Depending on a number of factors, such as material and color, for example, a shift occurs in the resonant frequency of the radiator for the antenna, thereby affecting its transmitting and receiving power. Furthermore, in particular for cell phones, numerous exchangeable housing parts such as face plates, back shells, or battery covers are offered to customize the design of the device. In addition to various colors, frequently created using metallized paints or metal-filled plastics that affect an antenna's tuned frequency, there are also housing parts made of a variety of materials, such as of plastic with additional leather or fabric applications.
A method not documented in the published prior art for providing an antenna that tolerates such variable circumstances consists in the use of antennas having a large bandwidth and consequently a larger size. However, this conflicts with the above-mentioned requirements for increasingly smaller components, in addition to smaller antennas.
It is also known from U.S. Pat. No. 6,680,703 to provide a system of several varactors to allow tuning of a narrow-band antenna. Such a system does indeed allow a technician to fix a detuned antenna, but is not readily applicable, and is fairly expensive because of the active circuit elements used, that is the varactor diodes. It is not usable, for instance, in a cheap replacement back shell that is going to be installed by a technically unsophisticated user, and that might be purchased solely to give the electronic device in question a certain decor.